The present invention concerns in general terms a method of transmitting data over a physical resource. More precisely the present invention concerns a transmission method combating the degradation of the quality of service in a mobile telecommunications system.
Such a system consists of one or more receivers connected to one or more transmitters by radio links. The transmission capacity of such a system is limited by the noise level, the interference between the signals of the different users, the available bandwidth etc. It must therefore be shared between the different users according to a resource allocation process enabling each user to have available as far as possible the services which he has requested.
For this purpose, a mobile telecommunication system uses three protocol layers as depicted in FIG. 1.
The upper layer or RRC (Radio Resource Control) layer is responsible for managing the whole of the physical resource and guaranteeing the quality of service (QoS) for the services of the different users. The quality of services defines in particular the maximum routing time and the error rate of the service data units, also referred to as SDUs.
The layer 2 is divided into an RLC (Radio Link Control) sub-layer and an MAC (Medium Access Control) sub-layer.
The RLC sub-layer is responsible for supplying, to the different applications, a transmission support in accordance with the quality of service guaranteed by the RRC layer. The RLC sub-layer can operate according to different modes. The first, so called transparent, mode (TM) is itself divided into two sub-modes. According to the first sub-mode, the RLC can segment the SDUs in order to generate data packets or transmission units known as RLC PDUs (RLC Protocol Data Units). According to a second sub-mode, this segmentation is not enabled and an RLC PDU must correspond to an SDU. In a second mode, known as unacknowledged mode (UM), it can not only segment the SDUs but also concatenate the fragments thereof in order to generate the RLC PDUs. In a third mode, known as acknowledged mode (AM), it can also request the retransmission of the erroneous RLC PDUs. In particular, when it is functioning in acknowledged mode, the RLC layer is responsible for the retransmission of the erroneous units in which the errors have not been able to be corrected by the physical processing. This retransmission is effected once again in the form of RLC PDU (re)transmission units.
The structure of an RLC PDU is illustrated in FIG. 2. It is in the form of a packet comprising a useful load consisting of service data coming from one or more SDUs (according to the operating mode of the RLC sub-layer), a header supplying the signalling information necessary to the RLC layer and an error detecting code (CRC) for detecting whether or not the packet is erroneous. In the majority of mobile telecommunication systems offering an acknowledged mode, the size of the RLC PDU transmission unit is fixed throughout the duration of the connection. This is the case in particular with the third-generation mobile telephony system (UMTS) as specified in the 1999 version of the standard.
The MAC sub-layer is responsible for access to the shared physical resource. If a user wishes to have several services available simultaneously, the MAC sub-layer distributes these services over the resource reserved by the RRC layer. Access to the resource is divided into transmission time intervals (TTI). For a given service, the MAC sub-layer can send a number NP of packets, all of the same size TP, during a transmission time interval. However, the MAC sub-layer is not able to choose any values for NP and TP. The possible values of NP and TP are imposed by the RRC layer and only certain combinations of these values are authorised by the latter. The possible combinations are supplied by the RRC layer to the MAC sub-layer.
The layer 1 or physical layer PHY is responsible for the physical processing of the data. It includes in particular an error correcting coding/decoding function which enables it correct the major part of the errors affecting the data transmitted and to provide a low residual error rate to the upper layers. For example, in the UMTS system, the coding/decoding used for the services benefiting from the acknowledged mode of the RLC layer is a turbocoding/decoding. The processing unit of the physical layer being the transmission time interval TTI, the data in the packets to be transmitted are interleaved within this interval, which results in a uniform distribution of the residual errors over the duration of the TTI.
In a mobile telecommunication system of the code distribution multiple access (CDMA) type such as that of the UMTS, the transmission powers of each of the signals transmitted (by the base stations or by the mobiles) must be adjusted so that the level of the signal to noise ratio plus interference attains the level required by the quality of service. For a given receiver, the signal to noise ratio plus interference is defined as being equal to the ratio between the signal level received and the background noise level plus the level of total interference received.
However, as the transmission powers are limited, it may happen that the resource allocation system cannot guarantee the quality of service by increasing the transmission power of the corresponding signal. In such a case, the bit error rate increases substantially and the residual error rate on the packets (BLER) may exceed an acceptable threshold. Since each erroneous packet gives rise to a retransmission request, the effective transmission rate then tends towards zero.
In order to deal with such a congestion phenomenon, various solutions have been proposed such as that of blocking certain calls or greatly reducing the transmission rate of some transmitters. These solutions are however not satisfactory since they do not guarantee that the data routing time does not exceed the maximum time fixed by the quality of service.